1. Field of the Invention
The present invention relates to compressors used in heating, ventilation, and air conditioning (HVAC) systems and, more particularly, to a system for heating compressors in an HVAC system.
2. Description of the Related Art
A compressor of a heating, ventilation, and air conditioning (HVAC) system requires a lubricant to protect internal surfaces operating under high loads from contacting each other. The lubricant in the compressor is a mixture of oil and refrigerant that is used in a cooling or heating cycle of the HVAC system. Oil typically remains within the compressor, where it is most useful, but small amounts are carried over into refrigerant lines, the condenser, and the evaporator of the HVAC system.
At the end of the cooling cycle, some refrigerant may migrate to the compressor, where it is absorbed by oil in the compressor sump. When the compressor is started (“start-up”), an abnormal start-up condition, commonly referred to as vapor compression lock-up or (“VCL”), may occur. One contributing factor to a VCL event is dilution of oil in the compressor sump due to refrigerant migration.
In a VCL event, the pressure in the crankcase drops suddenly at start-up, causing the refrigerant in the compressor sump to flash to a vapor. The crankcase pressure will then rise, rapidly releasing refrigerant and lubricant into the discharge line of the compressor. As this occurs, the compressor is also pushing refrigerant through the condenser coil to generate high pressure liquid refrigerant, needed to open the thermal expansion valve (“TXV”) to the evaporator. Due to the relatively low internal volume of the condenser coil, the sudden surge of refrigerant and oil from the crankcase causes a back-up of refrigerant at the discharge line, increasing pressure.
When the refrigerant absorbed in oil flashes to a vapor at start-up, a foam comprising oil diluted by refrigerant vapor rises into the moving parts of the compressor. As a result, the lubricating ability of the oil is reduced and metal-to-metal contact of compressor parts can occur, until the refrigerant is sufficiently removed from the oil. Furthermore, oil pushed into the discharge line and into the rest of the system may deprive the compressor sump of a reservoir of oil sufficient to lubricate the compressor, which further contributes to the problems caused by VCL.
The sudden increase in pressure from refrigerant at the discharge line may trip a high pressure sensor, causing the HVAC unit to become inoperable, until the sensor is reset. Condensers configured with micro-channel condenser coils are more vulnerable to VCL, because the lower internal volume slows the rate at which refrigerant may flow through the coil, increasing the pressure at the compressor discharge line.
To lessen the likelihood of a VCL event in conventional HVAC systems, heaters are mounted to the crankcase of the compressor to increase the temperature of the compressor sump, during times when the HVAC unit is not operating. Increasing the temperature of the compressor sump forces refrigerant away from the compressor and increases the amount of refrigerant in the condenser. At start-up, the compressor operates as intended, pumping high pressure vapor refrigerant to the condenser and facilitating heat exchange.
The crankcase heaters have a relatively low wattage rating, e.g. 100 W for a compressor of 5 ton capacity. The low wattage necessitates that the crankcase heaters be on continuously when the compressor is off in order to keep refrigerant away from the compressor.
VCL may also occur the first time the HVAC unit is started after installation. Standard operating procedure is to turn on the crankcase heaters about 24 hours prior to the start-up time of the compressors.
What is needed are HVAC systems and methods that will improve the reliability and efficiency of HVAC units, reducing down time for maintenance and repair, and extending the life of the unit.